The field of the present invention relates to location services. More particularly, the field of the present invention relates to locating a mobile device by noting times of arrival of signals that travel between the mobile device and a plurality of transmitters or receivers located at known positions, and computing a location based on the noted times of arrival.
Recently, the FCC mandated the introduction of location services that can accurately locate wireless subscribers on all wireless networks. Two previously proposed approaches for implementing such location services are the xe2x80x9cuplinkxe2x80x9d approach and the xe2x80x9cdownlinkxe2x80x9d approach.
In the uplink approach, the mobile device (also referred to as a xe2x80x9chandsetxe2x80x9d or xe2x80x9cremote terminalxe2x80x9d) that is to be located sends out a signal such as a random access channel (RACH) burst. The time of arrival (TOA) of the signal is determined at each of a plurality of location measurement units (LMUs), together with an associated quality indicator ("sgr"). Each of these noted TOAs and "sgr"s is then sent to a computer. The computer then uses conventional algorithms, which are well known to those skilled in the art, to determine the location of the mobile device based on the TOA and "sgr" determinations made by the LMUs and the known location of the LMUs.
One suitable conventional location algorithm uses a Taylor search to locate the intersection of two or more hyperbolas. Details of a such an algorithm can be found in xe2x80x9cStatistical Theory of Passive Location Systemsxe2x80x9d by D. J. Torrieri, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-20, No. 2, March 1984. This algorithm locates the mobile device based on three or more TOA readings (which are used to determine the distance from the mobile device to the LMU based on the speed c of the signal), the associated "sgr"s, and the known locations of each LMU.
In the downlink approach, each of a plurality of base stations (BTSs) sends a signal to the mobile device to be located, and the mobile device determines the TOA and "sgr" of each of these signals. These TOAs and "sgr"s are then transmitted to a remote computer, which implements a conventional algorithm to determine the mobile device""s location based on the TOAs and "sgr", similar to the uplink type systems. Alternatively, if sufficient processing power is available in the handset, the algorithm may be implemented in the handset. When the transmission frames of the BTSs are not synchronized, the downlink algorithms are somewhat more complex because the computer must obtain the relative time difference between each BTS transmission to calculate a location. This relative time information can be obtained using auxiliary receivers located at known locations to measure the TOAs of the signals from the BTSs, in a conventional manner.
In both uplink and downlink systems, three TOA measurements are sufficient to form a location estimate. The accuracy of the location estimate, however, is limited by the resolution of the TOA measurements. In addition, when noise and interference corrupt the TOA measurements used as inputs to the location algorithm, the accuracy of the location estimate is further reduced. Using more than three TOAs (e.g., using 4-7 TOAs) to form the location estimate can provide improved accuracy, as compared to estimates based on only three TOAs. Unfortunately, in order to include additional TOA measurements, it is often necessary to rely on TOA readings with poor quality communication links. The poor quality of these communication links can counteract some or all of the benefits provided by the additional TOA measurements. In certain circumstances, a location estimate based on four or more TOA measurements may be even worse than an estimate based on only three TOA measurements.
Multipath interference can be particularly problematic because the path traveled by a multipath signal includes reflections (e.g., when the signal reflects off of buildings, mountains, etc.). These reflections increase the length of the path traveled by the signal, with a resulting increase in the TOA, so that the TOA no longer matches the straight-line distance between the signal source and destination. The inaccurate TOA then distorts the location estimate produced by the location algorithm. This problem is compounded by the fact that multipath signals can be relatively strong, and may therefore be selected to form a location estimate in place of weaker signals that may actually be more accurate.